Acute histological effects of interstitial hyperthermia on normal rat brain.

Histological changes in the brains of Fischer rats at different times after interstitial heating with various thermal doses were studied. The brains, subjected to sham-heating, and heating at 39 and 40 degrees C for 30 min showed mild capillary congestion and minimal vacuolation at 4, 24 and 72 h. In the brains heated to 41, 42 and 43 degrees C for 30 min, there was local vascular congestion, petechiae, vacuolation and cellular shrinkage with nuclear pyknosis at 4 h; enhanced congestion and petechiae, acute cellular necrosis, infiltration of polymorphonuclear leukocytes and marked vacuolation at the margin at 24h; total coagulative necrosis of all parenchymal and vascular elements, early liquefaction necrosis and vascular hyperplasia at the margin at 72 h; enhanced vascular hyperplasia at the margin at 120 h and 168 h. The threshold thermal dose for the histopathological damage in the rat brain was heating at 41 degrees C for 30 min.